The present invention relates generally to an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules and, more particularly, to a method and apparatus for exposing the imaging sheets with image information.
U.S. Pat. Nos. 4,440,846 and 4,399,209 describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is imagewise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and imagewise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet developing processing solutions to produce the image. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former imagewise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing imagewise exposed imaging sheets through the nip formed between a pair of parallel calendar rolls.
The media may exist in either single-sheet or two-sheet versions. In the former case, the microcapsules and developer composition are both coated onto a single substrate layer. In the latter case, the microcapsules are carried on a first substrate layer referred to as a donor sheet. The developer composition is coated onto a second, separate substrate layer referred to as a receiver sheet. The donor sheet is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheet. The two sheets are then developed by pressure, with the finished image being formed in the receiver sheet.
The disclosed imaging system may be designed to produce color images. In such case, three or more different types of capsules may be present on the sheet. Each is responsive to a different wavelength of exposure radiation and contains color formers designed to produce different colors upon subsequent development. Disclosure of such systems is made in U.S. Pat. No. 4,576,891 and U.S. Pat. No. 482,976.
Exposure of a color media can be made complicated due to differing sensitivities of the microcapsules corresponding to the different color. Thus, without some means of compensation, the color balance in the produced image may differ considerably from the original image.
U.S. patent application Ser. No. 262,545 filed Oct. 25, 1988, discusses a phenomenon exhibited by the imaging media known as short time scale reciprocity failure. Absent this effect, it would be expected that the imaging media would exhibit a substantially linear relationship between exposure time and exposure intensity. However, at short exposure times, the media exhibits a dramatic loss in sensitivity. This effect is also complicated in a color system, since the microcapsules for the different colors may exhibit reciprocity failure beginning at different threshold intensities, and may exhibit such effect to differing degrees.
What is needed, therefore, is a system for exposing the microencapsulated media with image information, and particularly for exposing a color microencapsulated media to color image information. Such exposure system should be capable of manipulating the color balance of the image information in accordance with the requirements of the media, and should be capable of minimizing the effects of short time-scale reciprocity failure.